MOKPHOLOGY OF CYCLOSTOMATA. O/O 



respectively. The venous blood is carried away tlirongli the an- 

 terior cardinal veins which penetrate the pronephros. 



20. These Idood-spaces are thus segmental in arrangement 

 '<\m\intn'xoiintic in position. The two anterior pairs of them soon 

 nndo]-go atrophy, Imt the posteriormost pair persists, becoming 

 enlaro-ed and sacculated at the distal extremitv. This sacculated 

 part of the vessel is filled up with free-cells and is called the 

 glomerulus of the pronephros, and, therefore, there is only a pair 

 of glomeruli in Petromyzon. 



II. Historical Review and Conclusions. 



As is well known, ]Max Schultze ('-56) was the first who dis- 

 covered the pronephros in Petromyzon. Having investigated the 

 larvœ of P. lyhineri, the author describes the structure as 

 "Drüsenanlage" and homologised it with the "Urnieren (Wolf'sche 

 Körper) " of the frog's larva. His statements on this body are as 

 follows : " Nicht lange nach der Bildung dieser Drüse (Thymus) 

 entstellt die Anlage einer zweiten, aus dem unter der Chorda 

 dorsalis ans-ehäuften Blastem über dem Herzen. Aus der durch 

 Pigmentanlagelungen früh schon sehr undurchsichtig w^erdenden 

 Masse wachsen nämlich nach unten, gegen das Herz zu, 3 oder 

 4 kurze Fortsätze hervor, welche eine eigenthüraliche Wimperung 

 zeigen" (p. 30). 



The stage spoken of probably corresponds to Stage v, or vi, 

 of my embryo. 



Our knowledge on this subject received important additions 

 by the noted investigations of W. Müller and Max Für- 

 BRIXGER. MuLLER ('75) uoticcd the first traces of the pro- 

 nephros in a very young embryo, which had yet only four pairs 



